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SubjectRe: [PATCH v2 1/4] tracing: Remove usage of list iterator after the loop body
On Sat,  2 Apr 2022 12:33:38 +0200
Jakob Koschel <jakobkoschel@gmail.com> wrote:

> In preparation to limit the scope of the list iterator variable to the
> traversal loop, use a dedicated pointer to point to the found element
> [1].
>
> Before, the code implicitly used the head when no element was found
> when using &pos->list. Since the new variable is only set if an
> element was found, the head needs to be used explicitly if the
> variable is NULL.
>
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
> ---
> kernel/trace/trace_output.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 8aa493d25c73..733a4d6c20e2 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -692,7 +692,7 @@ static LIST_HEAD(ftrace_event_list);
>
> static int trace_search_list(struct list_head **list)
> {
> - struct trace_event *e;
> + struct trace_event *e = NULL, *iter;
> int next = __TRACE_LAST_TYPE;
>
> if (list_empty(&ftrace_event_list)) {
> @@ -704,9 +704,11 @@ static int trace_search_list(struct list_head **list)
> * We used up all possible max events,
> * lets see if somebody freed one.
> */
> - list_for_each_entry(e, &ftrace_event_list, list) {
> - if (e->type != next)
> + list_for_each_entry(iter, &ftrace_event_list, list) {
> + if (iter->type != next) {
> + e = iter;
> break;
> + }
> next++;
> }
>
> @@ -714,7 +716,10 @@ static int trace_search_list(struct list_head **list)
> if (next > TRACE_EVENT_TYPE_MAX)
> return 0;
>
> - *list = &e->list;
> + if (e)

Technically, if e was NULL, then next would be too big. But as that's very
subtle with the algorithm, I'm fine adding this.

-- Steve


> + *list = &e->list;
> + else
> + *list = &ftrace_event_list;
> return next;
> }
>

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